The invention relates to a method of producing a calcined raw meal and of producing additional steam in an existing steam generator.
In the production of Portland cement, the raw ingredients after having been preheated pass through different heating stages in the clinker making process. Thus in a rotary kiln they first pass through a calcining zone in which calcium carbonate CaCO3 is thermally decomposed to lime. The calcining zone is typically at temperatures over 800xc2x0 C. The development of rotary kilns for clinker production is directed to the goal of utilizing the shortest possible kiln which leads to an kiln-external precalcining of the raw mix. Or in such precalciners arranged upstream the feed end of a kiln, the concentration of alkali and sulfur compounds can lead to the formation of injurious incrustrations. Under some conditions, dust and material circulations can interact when the basic flow is disrupted due to the incrustrations or lining damage. As a result, continued circulation can lead to high build-up concentration of compounds at individual locations of the calcining system even though these materials are present in only small amounts in the raw mix or in the fuel.
On the other hand, a considerable number of power plants, especially of the PC-type, have a utilization factor, which is lower than their availability. Say a power plant, which has an availability of normally 85-90%, operates only with a utilization factor of 60%. This can be the case for old plants, which have been not equipped with environmental systems and are therefore used only to provide the peak power demands, or are used as stand-by plants. Old plants are also used part time, because their efficiency is lower as compared to the newer ones.
Accordingly, the object of the invention is to provide a novel method to revamp an existing fossil power plant being equipped either by a pulverized coal fired boiler or by a circulating fluidized bed boiler, while simultaneously providing a product that might be used in the cement industry.
This is achieved, according to the invention, by following method steps:
pulverized raw meal, hydrocarbon, primary air and secondary air are fed to a calciner system, wherein the raw meal is calcined in a temperature range between 850xc2x0 C. and 950xc2x0 C.,
the calcined raw meal is separated from the calciner flue gases at the exit of the calciner system,
the calcined raw meal is directed through a cooler, in which it exchanges its heat with the secondary air,
the calciner exhaust gases are directed in the gas path of the backpass of the steam generator at an appropriate temperature window and are used for steam production.
The advantages of the invention are to be seen in that the waste heat from the calcining flue gases can be used for steam production by cooling the gases in the backpass of the boiler without impacting the boiler operation and design. If limestone is calcined to lime in the calciner system, the cement industry is provided with a product that enables an increase of about 100% of clinker output with an existing rotary kiln. If the steam is used for power production, other advantages are to be seen in a high level of operation fexibility of such an electricity/lime co-production system; the possibility of autonomous or highly independent control of power generation and calcination processes is given. Furthermore existing power plants which operate at low capacity factor are cost effectively upgraded to produce a second revenues stream.
During introduction of the exhaust gases into the backpass of the steam generator, the fuel feed to the steam generator may be reduced. This feature has the advantage of producing steam at lower cost.
During introduction of the calciner exhaust gases into the backpass of the steam generator, the feed of the sulfur sorbent material to the steam generator may be ceased if a circulating fluidized bed boiler is used, or the existing sulfur removal system may be put out of operation if a boiler with pulverized coal is used. This feature results in considerable savings, since the desulfurization of the boiler flue gases is now performed by the lime contained in the calciner exhaust gases. Thus specially dedicated desulfurization systems may be substituted.